While there is a certain grand mystery to some aspects of human behavior, others can be easily explained. Just find yourself a garden-variety house cat, along with a $10 laser pointer.

Many cat owners know that the lasers are the easiest way to keep the pet amused. The cats will ceaselessly, maniacally chase it as it's beamed about the room, literally climbing the walls to capture what they surely regard as some form of ultimate prey.

Obviously, cats are hard-wired to hunt down small, bright objects, like birds. But since nothing in nature is as bright as a laser, they are powerless to resist its charms.

Cats and lasers are useful in explaining some of the more addictive aspects of Web use, including a recent occurrence on the site for Andrew Sullivan, a popular political blogger. Mr. Sullivan's blog doesn't follow the standard practice of making room for readers to add their own comments after each blog item. Curious if he should change his policy, he put the question to a vote.

Readers responded 60-40 against allowing comments. Even more striking than the fact that these readers were denying themselves a voice was the reason some of them gave for declining the offer: Like cats chasing a laser, they wouldn't be able to stop themselves.

"In truth we would rarely opt not to read them," said one reader. "Blog comments have the power to hammerlock one's attention. ... We'd be impotent to resist looking over the rantings and counter-rantings. ... Not only would comments be an incredible drain on one's time (especially if we check your blog several times a day from work), but it also exposes readers to the nasty underbelly of blogging."

What is it about a Web site that might make it literally irresistible? Clues are offered by research conducted by Irving Biederman, a neuroscientist at the University of Southern California, who is interested in the evolutionary and biological basis of the human need for information.

Dr. Biederman first showed a collection of photographs to volunteer test subjects, and found they said they preferred certain kinds of pictures (monkeys in a tree or a group of houses along a river) over others (an empty parking lot or a pile of old paint cans).

The preferred pictures had certain common features, including a good vantage on a landscape and an element of mystery. In one way or another, said Dr. Biederman, they all presented new information that somehow needed to be interpreted.

When he hooked up volunteers to a brain-scanning machine, the preferred pictures were shown to generate much more brain activity than the unpreferred shots. While researchers don't yet know what exactly these brain scans signify, a likely possibility involves increased production of the brain's pleasure-enhancing neurotransmitters called opioids.

In other words, coming across what Dr. Biederman calls new and richly interpretable information triggers a chemical reaction that makes us feel good, which in turn causes us to seek out even more of it. The reverse is true as well: We want to avoid not getting those hits because, for one, we are so averse to boredom.

It is something we seem hard-wired to do, says Dr. Biederman. When you find new information, you get an opioid hit, and we are junkies for those. You might call us 'infovores.' "

For most of human history, there was little chance of overdosing on information, because any one day in the Olduvai Gorge was a lot like any other. Today, though, we can find in the course of a few hours online more information than our ancient ancestors could in their whole lives.

Just like the laser and the cat, technology is playing a trick on us. We are programmed for scarcity and can't dial back when something is abundant.

The same happens with food: Because at one time we never knew when the next saber-toothed tiger might come along for food, it made sense to pack on the calories whenever we chanced upon them. That's not much help in today's world of snack aisles and super sizes.

Using computers traditionally has been associated with Mr. Spock-style cerebration, the ultimate kind of left-brain activity. But Dr. Biederman is just one of many researchers now linking it with some of the oldest parts of the human brain.

A group of StanfordUniversity researchers, for example, recently found gender differences in the brains of computer gamers. Males showed more neural firings, suggesting that they were physically experiencing the game in a manner different from women.

Watching a cat play with a laser, you realize the cat never learns there is no real "prey" there. You can show the cat the pointer, clicking it off and on, and it will remain transfixed.

Indeed, while cats find a causal link between the pointer and the shimmering light, they come to a wrong conclusion. They believe the pointer is the container that holds the prey, and that the critter is released once the cat's owner gets the pen down from the shelf and starts to wave it around.

People presumably are smarter than cats, and as we become more familiar with the Web and its torrent of information, maybe we'll do a better job learning what is useful and what isn't.
"Some people will abuse alcohol -- driving drunk, for example -- but they only drink heavily once a month. They can remain stable for a long time and not progress to dependence," says Mark L. Willenbring, director of the division of treatment and recovery research at the NIAAA. "And people can be dependent and not have abuse problems at all. They're successful students. They're good parents, good workers. They watch their weight. They go the gym. Then they go home and have four martinis or two bottles of wine. Are they alcoholics? You bet. And the goal is to get treatment for these folks, earlier, that is acceptable and attractive and effective."

To that end, some experts want the DSM-V -- the new edition now being compiled -- to combine abuse and dependence into a single "alcohol-use disorder" that ranges in severity, taking into account harmful drinking patterns and other symptoms. The aim is for simmering problems to be spotted sooner.